Contributions to Industrial-Minerals Research

U.S. Department of the Interior

U.S. Geological Survey

Bulletin 2209–B

Regional Geologic Setting of Late Cenozoic Lacustrine 

Diatomite Deposits, Great Basin and Surrounding 

Region: Overview and Plans for Investigation

Chapter B of
Contributions to Industrial-Minerals Research

By Alan R. Wallace

Bulletin 2209–B

U.S. Department of the Interior

U.S. Geological Survey 

Regional Geologic Setting of Late Cenozoic Lacustrine 

Diatomite Deposits, Great Basin and Surrounding 

Region: Overview and Plans for Investigation

James D. Bliss, Phillip R. Moyle, and Keith R. Long, Editors

Chapter B of
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

Version 1.0, 2003

This publication is available only online at:

http://pubs.usgs.gov/bul/b2209-b/

Text edited by George A. Havach

Layout by Stephen L. Scott

Manuscript approved for publication, July 10, 2003

Any use of trade, product, or firm names in this publication

is for descriptive purposes only and does not imply 

endorsement by the U.S. Government 
CONTENTS

Abstract 1

Introduction 1

Regional Geologic Setting 1

Tectonics 1

 Volcanism 2

 Climate 3

Late Cenozoic Lacustrine Basins 3

 Overview 3

 Ages and Distributions 3

 Basin Characteristics 5

Diatomite Deposits and Diatoms 6

 Overview 6

 Diatom Taxonomy 6

Deposit Characteristics 6

 Paleoecology and Formation 7

 Duration of Diatomite Formation 8

Postdepositional Events 8

 Overview 8

 Cover Units 8

 Diagenesis and Alteration 9

 Tectonic Activity and Erosion 9

Conclusions and Future Plans 10

Acknowledgments 10

References Cited 10

Figure

1. Western United States, showing locations of diatomite deposits 2

2. Regional late Cenozioc geologic setting of the Great Basin region 3

3. Western United States, showing distribution of Late Cenozoic sedimentary 

 rocks (blue areas) 4

III
1

Abstract

Freshwater diatomite deposits are present in all of the 

Western United States, including the Great Basin and surrounding regions. These deposits are important domestic 

sources of diatomite, and a better understanding of their formation and geologic settings may aid diatomite exploration 

and land-use management.

Diatomite deposits in the Great Basin are the products of 

two stages: (1) formation in Late Cenozoic lacustrine basins 

and (2) preservation after formation. Processes that favored 

long-lived diatom activity and diatomite formation range 

in decreasing scale from global to local. The most important global process was climate, which became increasingly 

cool and dry from 15 Ma to the present. Regional processes 

included tectonic setting and volcanism, which varied considerably both spatially and temporally in the Great Basin 

region. Local processes included basin formation, sedimentation, hydrology, and rates of processes, including diatom 

growth and accumulation; basin morphology and nutrient 

and silica sources were important for robust activity of different diatom genera. Only optimum combinations of these 

processes led to the formation of large diatomite deposits, 

and less than optimum combinations resulted in lakebeds that 

contained little to no diatomite.

Postdepositional processes can destroy, conceal, or preserve a diatomite deposit. These processes, which most 

commonly are local in scale, include uplift, with related erosion and changes in hydrology; burial beneath sedimentary 

deposits or volcanic flows and tuffs; and alteration during diagenesis and hydrothermal activity. Some sedimentary basins 

that may have contained diatomite deposits have largely been 

destroyed or significantly modified, whereas others, such as 

those in western Nevada, have been sufficiently preserved 

along with their contained diatomite deposits.

Future research on freshwater diatomite deposits in the 

Western United States and Great Basin region should concentrate on the regional and local processes that led to the formation 

and preservation of the deposits. Major questions that need to 

be answered include (1) why were some basins favorable for 

diatomite formation, whereas others were not; (2) what postdepositional conditions are needed for diatomite preservation; 

and (3) what were the optimum process combinations that led to 

the formation and preservation of economic diatomite deposits?

Introduction

Late Cenozoic lacustrine diatomite deposits are common 

in parts of the Great Basin region and other Western States 

(fig. 1); they are second only to large marine diatomite deposits in southern California as a diatomite resource in the United 

States. Diatomite is or has been mined from lacustrine deposits in all of the Western States, of which Nevada currently is 

the largest producer.

Sedimentary basins were present in the Great Basin 

and surrounding regions at various places and times during 

the past 16 m.y. Many of these basins contained small to 

extensive lakes, in some of which the conditions favored the 

formation of thick diatomite deposits in the lakebeds. Other 

contemporaneous basins in the region contain only little or 

no lacustrine sedimentary materials or diatomite deposits. 

In addition, postdepositional processes, such as volcanism, 

sedimentation, uplift, erosion, and various forms of alteration, 

varyingly combined to preserve or destroy diatomite deposits 

and host sedimentary rocks.

Getting from a sedimentary basin in the past to a 

diatomite deposit in the present requires processes that both 

formed and preserved the deposit. The focus of the new 

Industrial Minerals in Lacustrine Environments Task of the 

Western Industrial Minerals Project (WIMP) is to evaluate 

these events and processes and to develop a genetic model 

that can be used for exploration and mineral-resource assessments. This chapter presents a brief overview of Late Cenozoic lacustrine basins in the Great Basin region of the Western 

United States, and it introduces various processes that may 

have contributed to the formation and preservation of diatomite deposits. The ideas presented and questions posed here 

will be evaluated in more detail during the multiyear course 

of this study.

Regional Geologic Setting

Tectonics

The effects of Late Cenozoic tectonism varied spatially 

and temporally throughout the Great Basin region. In some 

parts of the southern and eastern Great Basin, late Oligocene 

crustal extension continued into the early to middle Miocene. 

Regional Geologic Setting of Late Cenozoic Lacustrine 

Diatomite Deposits, Great Basin and Surrounding 

Region: Overview and Plans for Investigation

By Alan R. Wallace

Regional Geologic Setting
2 Contributions to Industrial-Minerals Research 

This extension led to the formation of extensive detachment 

structures in the Grant, Snake, and East Humboldt Ranges 

and the Ruby Mountains in eastern Nevada, as well as other 

structures related to low-angle faulting in southeastern 

Nevada (Axen and others, 1993). Most of this faulting ceased 

or was waning by about 15 Ma.

In many parts of the Great Basin region, west-southwestward-directed crustal extension began in the middle Miocene 

and continued into the late Miocene, possibly as late as 6 Ma 

(fig. 2; Zoback and Thompson, 1978). This crustal extension 

created abundant high-angle faults and small to moderate 

uplifts separated by broad basins. At about 6 Ma, the extension direction shifted northwestward (fig. 2; Zoback and 

Thompson, 1978; Christiansen and Yeats, 1992). High-angle 

faults that formed between 6 Ma and the present produced the 

modern horst-and-graben topography typical of much of the 

Great Basin.

In western Nevada, right-lateral movement began along 

the Walker Lane structural zone as early as 12 Ma (fig. 2; 

Hardyman and Oldow, 1991; Stewart and Perkins, 1999; 

Schwartz, 2001). This right-lateral movement, in combination 

with the post-6-Ma northwestward-directed crustal extension, has caused oblique movement along faults in the Walker 

Lane, in contrast to crustal extension in the regions to the 

east that produced dip-slip normal faults (Henry and Faulds, 

2002).

Possible topics of study:

•What tectonic environments were most conducive to the 

formation of lacustrine basins?

•When and where did these basins form in the context of 

 the regional tectonics?

•How did tectonics affect the paleotopography, and how 

 did this effect of tectonics on topography change in 

 space and time?

Volcanism

Two regional volcanic systems (western andesite and 

bimodal) were active in the Great Basin and surrounding 

region during Neogene and Quaternary time. Early to late 

Miocene volcanism related to the early Cascades volcanic arc 

created the western andesite assemblage. This volcanic arc 

included stratovolcanoes and shield volcanoes that erupted in 

the western Great Basin and to the north-northwest beneath 

the modern Cascade volcanic chain (fig. 2).

Bimodal mafic-felsic volcanism influenced much of 

the north-central Great Basin (fig. 2), producing widespread 

mafic flows and more localized rhyolite domes and tuffs. 

Volcanism began about 17 Ma with the eruption of the 

voluminous and regionally extensive Steens Basalt in the 

northwestern Great Basin, the Columbia River Basalt Group 

in eastern Oregon and Washington, and other mafic flows 

elsewhere in the northern Great Basin. Bimodal volcanic 

activity waned after about 14 Ma but has continued weakly 

to the present. The Yellowstone hotspot, a thermal and magmatic plume, penetrated the crust near McDermitt, Oreg. (fig. 

2), inducing much of the bimodal volcanism in the northern 

Great Basin and surrounding areas. As the North American 

craton moved southwestward over the hotspot, a northeasttrending track of hotspot-related volcanism and faulting 

formed along what is now the Snake River Plain; currently, 

the hotspot is beneath Yellowstone National Park (fig. 2; 

Pierce and Morgan, 1992). Bimodal volcanism in the southern 

Great Basin produced widespread middle Miocene rhyolites 

and less extensive related mafic flows (fig. 2; Best and others, 

1989; Ludington and others, 1996), as well as Quaternary 

basalt flows and cones.

A third volcanic event, which formed the interior andesiterhyolite assemblage, swept southwestward through the Great 

Basin region between about 43 and 22 Ma. Although this event 

is older than the Late Cenozoic sedimentation and diatomite 

formation, the related volcanic rocks that are present near several of the sedimentary basins may have contributed sedimentary materials, silica, and nutrients to the lakes.

WA

ID

OR

CA

NV UT

AZ

Figure 1. Western United States, showing locations of diatomite 

deposits. Many of these deposits near the Pacific coast formed in 

marine environments; more interior deposits formed in freshwater 

environments. Data from U.S. Geological Survey (yellow squares) 

and U.S. Bureau of Mines (green circles); diatomite deposits in both 

data sets are shown by yellow squares only.

Figure 1 is an outline map of the Western United 

States: Washington, Oregon, Idaho, California, Nevada, Utah, and Arizona. Green circles and yellow 

squares show the locations of diatomite deposits. 

The figure caption follows
3

Figure 2 is an outline map of the Great Basin region during the Late Cenozoic Era, showing California, Nevada, and parts of Oregon, Idaho, Utah, and 

Arizona. The area of the map extends from latitudes 33 through 44 degrees 

north and from longitudes 112 through 125 degrees west. The areas of the 

Cascade Mountains in southern Oregon and California and the Sierra Nevada 

in California are shown in gray, and the Walker Lane is shown straddling 

the California-Nevada State line. Two areas of Late Cenozoic volcanic assemblages are shown: an early to late Miocene western andesite assemblage 

in light yellow, mostly around the Walker Lane; and a middle Miocene to 

Holocene bimodal assemblage in light blue, covering much of south-central 

and northern to northwestern Nevada, and extending off the map into eastern 

Oregon and southern and eastern Idaho. A red star indicates the location of 

McDermitt, and a northeast-trending red arrow denotes the trace of the Yellowstone hotspot from that point. The figure caption follows

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Possible topics of study:

• What volcanic systems were active during lacustrine 

 sedimentation, both regionally and locally?

• How did the volcanic systems affect the formation and 

 modification of the basins and lakes?

• Were specific volcanic forms or compositions more 

 influential on lake environments and diatomite forma-

 tion than others?

Climate

Global, regional, and local conditions affected the Late 

Cenozoic climate in the Great Basin and Western United 

States. About 17–14 Ma, west-central North America was 

warm and had moderate rainfall (Axelrod, 1956; Zachos and 

others, 2001). The climate then began to cool, and precipitation in the region gradually decreased into the Pliocene. The 

middle Miocene flora in the Great Basin region was similar 

to that along the modern western Sierra Nevada of California (Axelrod, 1956): conifers, pines, and mixed oak-birch 

woodlands were common, with marshes in lowlands (Schorn 

and Erwin, 2002). With cooling and drying, woodlands 

diminished, and grasslands and sagebrush expanded, eventually evolving into a more typical desert-scrub and grassland 

flora during the Pliocene (Davis and Moutoux, 1998; Retallack, 2001). Limited leaf-morphologic evidence suggests 

that the central core of the Great Basin region may have 

been 1 to 1.5 km higher than at present and that the elevation of the western margin was the same as today (Wolfe and 

others, 1997). Other researchers, however, have questioned 

the validity of this leaf-morphologic methodology (see Hamilton, 2002).

Regional and local events—primarily volcanism and 

tectonism—also influenced the climate of west-central North 

America during the Late Cenozoic. Eruption of the Steens 

and Columbia River basalt flows may have contributed significant CO2 to the atmosphere and been a factor in the global 

mid-Miocene climatic optimum (Zachos and others, 2001). 

Tectonism in various parts of the region created uplifts that 

interfered with the eastward-flowing weather patterns, much 

as the Sierra Nevada has blocked eastward-flowing weather 

patterns since the Pliocene (Henry and Perkins, 2001). Upliftrelated elevation changes likely influenced precipitation, erosion and sediment transport, and vegetation changes.

Possible topics of study:

• How is the progressive cooling and drying reflected in 

 Great Basin flora and fauna?

• What isotopic data (oxygen, deuterium) indicate cli-

 mate change, and what materials are most suitable for 

 study?

• How did local and regional climatic variations affect 

 runoff and nutrient inputs into lakes?

Late Cenozoic Lacustrine Basins

Overview

Remnants of Late Cenozoic sedimentary basins are 

scattered throughout the Great Basin region. The preserved 

sedimentary deposits demonstrate that lake-filled basins were 

common and, at times, widespread in the region throughout 

the Late Cenozoic. Some basins episodically have contained a 

lake from at least 15 Ma to the present (Davis and Moutoux, 

1998), whereas the lakes in most other basins had lifespans 

of less than 1 m.y. to several million years. In other basins, 

voluminous influxes of clastic materials prevented sizable, 

long-lived lakes from forming. Some, but certainly not all, 

lacustrine basins contain diatomite deposits of various sizes.

Ages and Distributions

Basins and lakes in the Great Basin region formed 

during several episodes in the Late Cenozoic (fig. 3). The 

Figure 2. Regional Late Cenozoic geologic setting of the Great Basin 

region. Two Late Cenozoic volcanic assemblages include early to 

late Miocene western andesite assemblage (light yellow) and middle 

Miocene to Holocene bimodal assemblage (light blue). Northeasttrending red arrow denotes the trace of Yellowstone hotspot from its 

~16.5-Ma inception near McDermitt (McD). Double-headed arrows 

in central Nevada denote different extension directions and their 

timing in the central Great Basin; opposite-pointing arrows along the 

Walker Lane show dextral strike-slip movement during the past 12 

m.y. Modified from John and others (2000).

Late Cenozoic Lacustrine Basins
4 Contributions to Industrial-Minerals Research 

igure 3 is an outline map of the Western United States: Washington, Oregon, Idaho, California, Nevada, 

Utah, and Arizona. Areas of Late Cenozoic sedimentary rocks are shown in blue. Small yellow circles 

shown the locations of diatomite deposits. Small red letters a through l denote the locations of major formations and terrestrial basins, to some of which are given general ages ranging from less than 1 to 16–15 

million years. The figure caption follows

Figure 3. Western United States, showing distribution of Late Cenozoic sedimentary rocks (blue areas). 

Quaternary sedimentary deposits are omitted except for playa deposits in Utah. Sedimentary deposits 

near the Pacific coast are largely marine (such as the Monterey Formation in California). Yellow circles 

mapped in figure 1. General ages of selected basins are based on studies cited in text. Major formations 

and terrestrial basins: a, Ellensburg Formation; b, Baker basin; c, Glenns Ferry/Chalk Hills Formations; d, 

Trout Creek Formation; e, Virgin Valley (Miocene) and Thousand Springs (Miocene and Pliocene) Formations; f, Humboldt and Carlin Formations; g, Horse Camp Formation; h, Truckee/Verdi/Chalk Bluff Formations; I, Stewart Valley/Middlegate basins; j, Panaca Formation; k, Quiburis Formation; l, Pine Valley (Hay 

Ranch Formation of Gordon and Heller, 1993).

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earliest lakes began to form about 16.5 Ma and were more 

widespread by about 15 Ma (Perkins and others, 1998). This 

period corresponds to the inception of early west-southwestward-directed crustal extension in the region and the early 

formation of shallow, broad basins. These lakes formed 

throughout much of the region. In eastern Washington and 

Oregon and northern Idaho, small to large lakes formed 

behind basalt-flow dams of the middle Miocene Columbia 

River Basalt Group and younger basalt flows (Smith and 

others, 1989). More localized lakes filled basins that were 

formed by coeval volcanic units, such as in the Trout Creek 

area of southern Oregon (Barrow, 1983).

A second period of lake formation took place about 13–7 

Ma, largely in western Nevada along the Walker Lane and in 

the Mojave region of southeastern California (fig. 3; Perkins 

and others, 1998). Lacustrine sedimentary materials contain 

some of the larger freshwater diatomite deposits in the region.

The youngest lakes formed during the late Miocene and 

Pliocene (fig. 3), and related lacustrine deposits are exposed 

in southeastern and northern Nevada and southwestern Idaho. 

Some of these lakes likely were precursors to the Pliocene 

and Pleistocene lakes that were extensive throughout the 

Great Basin (Reheis, 1999) and western North America.

Possible topics of study:

• What are the ages of the lacustrine sedimentary materi-

 als and diatomite deposits in the Great Basin?

• What was the duration of sedimentation in each basin?

Basin Characteristics

Evidence from basin-filling material reveals considerable regional variation in basin configuration, processes, 

and evolution. At one extreme are orogenic basins, such as 

the Horse Camp basin in eastern Nevada (g, fig. 3), which 

formed during detachment faulting in the middle Miocene. 

Coarse clastic debris derived from the adjacent, rapidly rising 

highland was shed into the developing basin, and lacustrine 

deposits accumulated only in the upper and distal parts of 

the section (Horton and Schmitt, 1998); diatomite was not 

formed. Similar middle Miocene orogenic basins are present 

at Sacramento Pass in eastern Nevada (Martinez, 2001) and in 

the eastern part of the Humboldt Formation northeast of Wells 

(Mueller and others, 1999).

At the other extreme are non-orogenic or early-orogenic 

basins, such as the western part of the middle Miocene Humboldt (Carlin) Formation basin in northeastern Nevada (f, fig. 

3). This part of the basin contained a relatively long lived, but 

at times ephemeral, lake between about 16 and 10 Ma (Sharp, 

1939; Smith and Ketner, 1976; Perkins and others, 1998; 

Wallace, 2003). Coarse clastic materials are far less common 

than near Wells, and airfall ash is a significant component of 

the basin-filling material, ranging in abundance from 5 to 10 

volume percent near Elko (Sharp, 1939) to nearly 100 percent 

at Ivanhoe (Wallace, 2003). Diatomite deposits are minor in 

this part of the basin, with only a small deposit near Carlin. 

The overall Humboldt (Carlin) basin represents a broad 

spectrum of depositional environments and processes. Other 

middle Miocene, early-extensional basins formed in the Stewart Valley and Middlegate areas of west-central and southwestern Nevada (i, fig. 3) and the in Virgin Valley Formation 

near McDermitt (e, fig. 3). Sedimentary deposits in those 

areas contain modest to significant amounts of diatomite 

(Axelrod, 1956; Barrow, 1983; Greene, 1984; Starratt, 1987; 

Stewart and others, 1999).

In eastern Oregon and Washington, extensive middle 

Miocene basalt flows of the Columbia River Basalt Group 

dammed drainages and formed lakes (Rember, 2002), such 

as the Ellensburg Formation in central Washington (a, fig. 3). 

With successive basaltic eruptions, existing lakes were covered, and new ones created (Smith and others, 1989; Retallack, 

2002). In eastern Oregon, middle Miocene rhyolite eruptions 

disrupted the drainage systems and formed locally extensive, 

diatom-bearing lakes (Barlock and Vander Meulen, 1991).

In western Nevada along the Walker Lane, thick, extensive remnants of middle to late Miocene basins are exposed 

from near Lovelock southward to Hazen and Yerington and 

westward to the Reno area (h, fig. 3). Sedimentation began 

in the basins at somewhat different times, ranging from 

about 13 Ma near Brady Hot Springs (southwestern Trinity 

Range) to about 6 Ma in the Wellington Hills Basin west of 

Yerington (Stewart and Perkins, 1999; Trexler and others, 

2000). Tephrochronologic studies indicate that several of 

these basins were active simultaneously (Stewart and Perkins, 1999), but whether they were connected is unknown. 

In all areas, the lakes eventually were succeeded by coarse 

alluvial systems related to incipient uplift of nearby horsts, 

or by volcanic flows. The diatomite deposits, several tens of 

meters to more than a hundred meters thick, that formed in 

these basins are the main focal point of the diatomite industry in Nevada and are mined in the Lovelock, Brady, Hazen, 

and Fernley areas.

Latest Miocene and Pliocene basins formed as the 

modern basin-and-range physiography began to develop 

about 6 Ma. The Pliocene Panaca Formation (j, fig. 3) is 

composed primarily of fine-grained fluvial, marsh, and 

pedogenic deposits, with smaller amounts of lacustrine 

deposits and diatomite near the center of the basin (Pederson 

and others, 2000). Volcanic rocks dammed the drainage to 

form the depocenter (Ekren and others, 1977); breaching 

of the dam during the Pliocene initiated a through-flowing 

drainage system. The late Miocene Glenns Ferry and Pliocene Chalk Hills Formations (c, fig. 3) formed in successive 

lakes, each of which were created by tectonic extension and 

damming by volcanic flows (Kimmel, 1982); both formations contain economic diatomite deposits. Other late Miocene and Pliocene basin sedimentary deposits are exposed in 

the late Miocene Thousand Springs Formation of northwestern Nevada (d, fig. 3; Greene, 1984) and the Pliocene Hay 

Ranch Formation of northeastern Nevada (l, fig. 3; Regnier, 

1960; Gordon and Heller, 1993), as well as isolated to more 

widespread basins in eastern Oregon and western Idaho. In 

Late Cenozoic Lacustrine Basins
6 Contributions to Industrial-Minerals Research 

addition, the basal fill beneath Quaternary alluvial fans in 

many major basins consists of late Miocene and Pliocene 

sedimentary materials, some of which contain lacustrine 

deposits (Madden-McGuire and others, 1991; McCarthy and 

Ehni, 2000).

Possible topics of study:

• How do the morphologies of Late Cenozoic sedimen-

 tary basins differ, and which morphologies were most 

 conducive to the formation of a quiet lacustrine envi-

 ronment?

• How did local tectonic and volcanic events affect basin 

 morphology, history, and contained materials?

• What was the space-time sedimentologic, hydrologic, 

 and geohydrologic context of each basin with regard to 

 the surrounding area?

• Were the basins connected or isolated?

Diatomite Deposits and Diatoms

Overview

On the basis of the brief summaries given above, diatomite deposits in the Great Basin region formed in some 

Neogene basins in response to different regional and local 

processes over the past 16 to 17 m.y. Although the general 

geologic setting of several of the deposits is known, detailed 

published descriptions of most of the deposits are sparse, and 

studies on Great Basin diatomite taxonomy and formation are 

limited.

Diatomite mines and prospects in the Western United 

States are mapped in figures 1 and 3. Most of the deposits 

shown are small, and development on those has been minor. 

Currently (2003) operating diatomite mines are concentrated 

in western Nevada, eastern Oregon, and central Washington, 

with mines in other States. All of these mines are near existing highways or railroads. Several diatomite deposits in western Nevada reportedly would be economically viable if they 

were closer to transportation infrastructure. Dolley and Moyle 

(2003) present additional information on diatomite production 

in the Western United States.

Diatom Taxonomy

Relatively few studies have been done on Late Cenozoic 

diatom taxonomy in the Western United States. The most 

common diatom genera in the diatomite deposits being mined 

in western Nevada are Melosira, Fragilera, and Cymbella 

(Krebs and Bradbury, 1984; Papke, 1992; Lenz and Morris, 

1993), and various other diatom genera are present as well. 

Melosira and Fragilera are the dominant genera in the Trout 

Creek basin in southeastern Oregon (d, fig. 3; Barrow, 1983). 

Navicula, Melosira, and Epithemia are the main diatom 

genera in the late Miocene to Pliocene deposits of southwestern Idaho (Moyle, 1985). Research by W.N. Krebs and 

J.P. Bradbury (Krebs and others, 1987; Bradbury and Krebs, 

1995; Krebs and Bradbury, 1995) focused on the taxonomy 

and regional distribution of the middle Miocene genus Actinocyclus. Although this genus is widespread in the region, it 

generally is not the major diatom genus in the deposits.

Possible topics of study:

• What diatom genera are present in Great Basin diato- 

 mite deposits, and in what relative abundances?

• Where in the deposits do these diatom genera occur, 

 and what is their relation to other sedimentologic fea- 

 tures and events?

• Economically, what diatom genera and species are 

 most important, and what conditions favored their 

 growth?

Deposit Characteristics

The few published reports on diatomite deposits in the 

Great Basin region indicate that the deposits vary considerably in form, size, and stratigraphy. All of these deposits 

contain diatomite beds or zones of varying thickness, and 

all contain varying amounts of clastic sedimentary material and air-fall ash. Higher-quality diatomite deposits, 

including those currently being mined, are thick (tens of 

meters to more than 100 m) and relatively free of contaminants.

The amount of interbedded material varies vertically and laterally through the diatomite deposits. Several 

deposits contain relatively pure diatomite near the center 

of the deposit but laterally contain increasing amounts of 

clastic material, indicating proximity to shorelines. Other 

variables that may influence diatomite quality include the 

position of inflowing streams relative to centers of diatomite formation or the amount of rainfall and transport of 

clastic sedimentary materials to the depocenter. Thin to 

thick clastic beds and (or) volcanic flows separate diatomite zones at the Quincy deposit in western Washington 

(a, fig. 3) and at the Hazen deposits in western Nevada 

(h, fig. 3). The interdiatomite beds indicate interruptions 

of, and then a return to, low-energy lacustrine conditions. 

Diatom genera commonly vary with depositional environment, indicating formation in different parts of the lake 

ecosystem. The presence or absence of ash could be due to 

the positions and sizes of eruptions that took place during 

diatomite formation, coupled with the wind direction, or 

to the recurrence rate and size of eruptions relative to the 

timespan of diatomite formation.

At several deposits, including the Trinity, Brady’s, 

and Hazen deposits in western Nevada (h, fig. 3), the diatomite-rich beds apparently occur near the base of the Miocene section and are covered by either clastic sedimentary 

materials or volcanic rocks, suggesting that lacustrine 

sedimentation took place during early-extensional-basin 
7

development. Other deposits, such as Trout Creek (d, fig. 

3; Barrow, 1983) and Chalk Hills (h, fig. 3; Schwartz, 

2001), formed in basins between volcanic eruptions, 

although at least 1 m.y. may have separated volcanism and 

diatomite formation. The Miocene and Pliocene Clover 

Creek Formation in southwestern Idaho (c, fig. 3; Moyle, 

1985) formed on two suites of Miocene basalt shortly 

after each suite was erupted.

Most of the diatomite deposits in the Great Basin 

region are small and represent relatively short periods of 

lacustrine sedimentation in restricted areas. Some of these 

deposits have been prospected, but most are undeveloped 

and so are not shown in figure 1. Diagenetic alteration is 

common in many of the deposits, possibly because the 

enclosing clastic sedimentary materials allowed ample 

interaction with ground and pore water.

Possible topics of study:

• Where and when did diatomite form within the space-

 time context of sedimentary and lacustrine basins?

• How do diatomite deposits fit into the sedimentologic 

 and hydrologic histories of the host basins?

• Is there a correlation between basin morphology and 

 the size of the contained diatomite deposit?

• What processes influenced the vertical and lateral 

 variations in the amount of clastic sedimentary materi-

 als in diatomite deposits?

• What short- and long-term effects did ashfalls and clas-

 tic deposition have on diatom activity?

• How did continued tectonic activity modify basin 

 dynamics and depositional environments during diato- 

 mite formation?

Paleoecology and Formation

Modern diatom studies use the ratio of pennate diatoms 

(such as Fragilera) to centric diatoms (such as Melosira) 

to show relative degrees of eutrophication (productivity) 

(Wetzel, 2001), which can be a function of changes over 

time (seasonal to longer) or position within a lake. Geologic 

relations in the Great Basin indicate that many lakes were 

shallow and eutrophic (as locally indicated by Fragilera species), but the abundance of Melosira species may indicate 

locally deeper and (or) less productive environments (Krebs 

and Bradbury, 1984). At the Hazen deposits, Melosira and 

Fragilera diatoms occur in different parts of the deposits, 

indicating temporal and spatial variations in water depth 

and eutrophy (Krebs and Bradbury, 1984). At Trout Creek 

(d, fig. 3), Barrow (1983) used alternating Melosira- and 

Fragilera-rich beds to infer seasonal changes in eutrophy. 

Plant, mammal, and diatom fossils indicate that marshes and 

wetlands rimmed many of the lakes in the region, although 

the abundance and types of these various organisms varied 

as the climate gradually became relatively drier through the 

Neogene (Davis and Moutoux, 1998; Retallack, 2001). Many 

of the lakes also supported abundant fish (Smith and others, 

1982; Brown, 1986; Smith, 1987), as well as algal buildups 

that formed thin to thick carbonate beds (Willden and Speed, 

1974; Smith and Ketner, 1976).

Water temperature, pH, chemistry, and nutrients influence diatom activity, and all of these parameters are influenced by a wide variety of processes. Limited studies on 

Great Basin diatomite deposits indicate that the lake waters 

were warm and had neutral to slightly alkaline pHs (7–8 and 

possibly greater), carbonate buffering, and low salinities 

and Na/K:Ca/Mg ratios, all of which favor diatom activity 

(Barrow, 1983; Sheppard and Gude, 1983; Lenz and Morris, 

1993). Barrow (1983) noted that the diatom species in the 

Trout Creek deposits indicate low salinities and a continual 

balance between lake inflow and outflow.

Since diatom frustules (the preserved amorphous cell 

walls of diatoms) are composed entirely of silica, the diatoms 

need dissolved silica in the water to flourish. Abundant silica 

was needed to form the large diatomite deposits in western 

Nevada. High diatom productivity can consume all available 

silica and eventually limit diatom activity (Wetzel, 2001). 

However, if viewed from a typical twice-a-year diatom bloom 

cycle, sufficient silica was needed only on a semiannual 

basis for the thousands of years it took to form the deposits. 

Silica sources likely included the volcanic highlands that surrounded the lakes and ashfalls on the lakes. As shown in the 

ash-rich Humboldt (Carlin) Formation, too much direct volcanic input may limit diatom activity by basically smothering 

the lake.

Phosphorus is the most important nutrient for diatom 

activity (Hall and Smol, 1999; Wetzel, 2001), and the cycle 

and bioavailability of phosphorus within lakes are complex. 

This element can be derived from many sources. In the Great 

Basin and much of the Western United States, runoff from 

nearby highlands is the most common source (Wetzel, 2001). 

Andesitic to basaltic volcanic rocks were common in the 

highlands near many of the large diatomite deposits in the 

Great Basin region, as well as at the basalt-related deposits 

in Idaho, Washington, and Oregon. Basalts and andesites 

contain, on average, far more phosphorus (1,400–1,600 

ppm) than do other igneous (170–700 ppm) and sedimentary (170–750 ppm) rocks (Krauskopf, 1979). Speculatively, 

phosphorus in seasonal runoff from intermediate-composition 

to mafic volcanic rocks may have favored diatom activity at 

many diatomite deposits in the region. However, rhyolitic 

volcanic rocks apparently dominated the highlands at a few 

large diatomite deposits, such as those in the Trinity Range 

near Lovelock (h, fig. 3; Nash, 1995), and volcanic-rock composition may not be the only factor in phosphorus source and 

availability.

The long- and short-term hydrologic balances between 

precipitation and evaporation, and among inflow, throughflow, and outflow, strongly affect the compositions and 

eutrophy of lakes. For example, drought decreases precipitation and runoff; in combination with continued evaporation, 

lakes can decrease in size and lose connections to outflow. 

As a result, eutrophy (Webster and others, 1996), elemental 

Diatomite Deposits and Diatoms
8 Contributions to Industrial-Minerals Research 

concentrations, and possibly, pH increase. Conversely, higher 

rainfall creates more input into the lake (including clastic 

materials and nutrient-rich runoff). The lake expands, connects with other basins, and may have a greater throughflow. 

All of these factors contribute to diatom activity and the formation of diatomite.

Possible topics of study:

• What were the short-term (seasonal to multiannual) 

versus long-term hydrologic and chemical changes during 

diatom blooms and diatomite formation, and how can they be 

determined and differentiated?

• What other flora and fauna provide evidence for the 

 paleoecology of diatom-rich lakes?

• What were the important sources of phosphorus, silica, 

 and other essential elements in diatomite deposits?

• Were any elements or environments toxic to diatoms?

• What are the essential processes that contribute to the 

 formation of a diatomite deposit, and what are their 

 relative importance?

• Similarly, what are the essential combinations of proc-

 esses that form a diatomite deposit?

Duration of Diatomite Formation

The general ages of most diatomite deposits in the Great 

Basin region are known from the ages of enclosing volcanic 

rocks. In the few places where tephras within the deposits 

have been dated, a specific age is known; however, how long 

it took to form the deposits is largely unknown. The duration of diatomite formation is constrained somewhat at the 

Brady’s deposits in western Nevada (h, fig. 3), where the base 

of the diatomite section was not dated but tephrochronology 

data suggest a duration of at least 600 k.y. (Stewart and Perkins, 1999). Additional 40Ar/39Ar ages and tephrochronology 

on interbedded ash layers and 40Ar/39Ar ages on bounding or 

interbedded volcanic flows could further constrain the duration of diatomite formation. The ages of bounding units, however, provide only maximum durations in the absence of other 

evidence of relation to lacustrine sedimentation. For example, 

many diatomite deposits overlie dated volcanic rocks, but 

the time between eruption and the start of sedimentation is 

unknown. The same holds true for overlying volcanic units 

and the end of sedimentation. Units bounding a large lacustrine basin in northernmost Nevada illustrate these points. The 

basal sedimentary deposits overlie Paleozoic, Eocene, and 

middle Miocene (15.8–16.1 Ma) rocks (Wallace, 1993; A.R. 

Wallace, unpub. data, 2000), giving an imprecise date for 

earliest sedimentation; this date would be even more imprecise were the Tertiary units not dated. In contrast, the capping 

15.1-Ma welded tuff contains conspicuous phreatic-eruption 

textures at its base, clearly indicating that the tuff was erupted 

into a shallow lake or wet sedimentary deposits and likely 

eliminated the lake environment (Wallace, 1993; A.R. Wallace, unpub. data, 2002). Thus, the sedimentary deposits were 

deposited between 15.8 and 15.1 Ma, but this estimate of 

duration is a maximum because of the imprecise date for the 

onset of sedimentation.

Many studies, both in marine and nonmarine environments, have demonstrated accumulation rates for diatoms in 

bottom sediment. These rates vary widely, depending on the 

paleoecology, productivity of the lake, climate, and many 

other factors. Depending on water chemistry, significant 

amounts of diatom frustules (max 90 percent) can dissolve 

during settling. Therefore, a direct comparison between diatomite thickness and the time needed for the deposit to form 

may be impossible in such fossil lakes as those in the Western 

United States.

Possible topics of study:

• How long did it take to form diatomite deposits in the 

 various basins?

• What processes controlled accumulation rates?

• How long were the diatomite-forming sedimentary and 

 lacustrine basins active, and during what stage(s) of 

 basin formation did the diatomite deposits form?

Postdepositional Events

Overview

Presuming that various processes led to the formation 

of a diatomite deposit in a lake-filled basin, preserving that 

deposit requires an optimum combination of other processes. 

Two major processes—alteration and erosion, either alone or 

in combination—can destroy a diatomite deposit. In addition, 

within a purely economic context, cover rocks must be minimally thick for the deposit to be discovered or economically 

viable.

Cover Units

Consolidated and unconsolidated materials cover most 

of the diatomite deposits in the Great Basin region, and the 

cover units include clastic sedimentary deposits and volcanic 

flows and tuffs. The cover units vary widely in thickness, 

from thin alluvial deposits to 1 km of volcanic flows. Some of 

the cover units were emplaced during or shortly after lacustrine sedimentation, whereas other cover units were deposited well after the lake environment ceased. Some younger 

lacustrine basins, such as the Panaca Formation and parts of 

the Pliocene deposits along the Snake River Plain, did not 

have significant cover units. In places, evidence suggests that 

younger units covered a diatomite deposit but that erosion has 

partly or largely removed them.

Cover units also provide information on the processes 

that ended lake sedimentation and that may have led to later 

alteration and erosion. The cover units that were emplaced 

during or immediately after diatomite formation effectively 
9

terminated the lake environment – for examples, in the Brady 

area, where coarse clastic sedimentary materials related to 

nearby incipient uplift buried the diatom-rich section (Stewart and Perkins, 1999), and the previously mentioned lake 

in northern Nevada, where a 15.1-Ma ash-flow tuff was 

erupted across the shallow lake (Wallace, 1993). At Ivanhoe 

in northern Nevada, the lakebeds (though without diatomite) 

were covered episodically by volcanic flows and air-fall ash 

and replaced by volcanic-related hydrothermal silicification 

(Wallace, 2003). Interdiatomite clastic sedimentary deposits 

and basaltic andesite flows near the Hazen deposits likely 

disrupted the lacustrine environment, followed by renewed 

lacustrine and diatom sedimentation. The capping basaltic 

andesite flows at the Hazen deposits are undated; if approximately contemporaneous with the upper diatomite beds, they 

may have ended the lacustrine sedimentation.

Possible topics of study:

• What types and thicknesses of cover units overlie dia-

 tomite deposits, and when did they form relatively to 

 the end of lacustrine sedimentation?

• What effect did these cover units have on preservation 

 of the deposits?

Diagenesis and Alteration

Diagenesis of lacustrine sedimentary material can 

begin almost immediately after deposition. Diagenetic and 

postdiagenetic products can include various cements, chert 

phases, zeolites, and clay minerals, many of which form at 

the expense of the original sediment and diatoms. Many of 

the recorded zeolite prospects in the central Great Basin are 

within Late Cenozoic sedimentary deposits. The amount 

and type of diagenetic alteration depend on several factors, 

including pore- and ground-water composition and the level 

of the ground-water table during lithification. In basins where 

diagenetic alteration was extensive, such as in one middle 

Miocene basin east of Paradise Valley in northern Nevada 

(Sheppard and Gude, 1983; Wallace, 1993), determination 

of the original presence or absence of diatomite may be difficult. Diatoms themselves undergo a structural transformation almost immediately after deposition, with a subsequent 

change to more stable silica phases.

Miocene and younger hydrothermal activity was localized around the many volcanic centers, and high heat flow in 

the northern Great Basin, especially in northern Nevada, over 

the past 6 m.y. has produced additional epithermal deposits 

and geothermal centers. Many of the volcanic and hydrothermal systems were emplaced in or adjacent to coeval lakes or, 

for young geothermal systems, in the sedimentary deposits 

of long-extinct basins. Although alteration related to these 

hydrothermal systems generally was not widespread, hydrothermal silicification and other alteration partly to completely 

replaced lacustrine sedimentary deposits in some basins 

(Ebert and Rye, 1997; Wallace, 2003). In coeval lakes and 

hydrothermal systems, subaqueous fluid venting may have 

contributed silica and, possibly, such elements as As, Hg, and 

Au into the lakes themselves.

Possible topics of study:

• What processes prevented or enhanced the formation 

 of diagenetic minerals in lacustrine sedimentary materi-

 als and diatomite deposits?

• What are the effects of subaerial exposure on diatomite 

 deposits?

• What effect did hydrothermal alteration have on lacus-

 trine sedimentary materials and diatomite deposits, and 

 what is the sphere of influence of an epithermal- or 

 geothermal-alteration halo?

Tectonic activity and erosion

Uplift related to Late Cenozoic northwestward-directed 

crustal extension affected various parts of the Great Basin 

region, and the amount of uplift and related erosion varied 

widely throughout the region. As a result, basins and lacustrine deposits in some areas retain much of their original 

dimensions, whereas a relict patchwork is all that remains 

in other areas. Such areas as the Snake River Plain and the 

Columbia River Plateau have undergone some crustal extension but relatively little uplift, and the original extents of the 

basins and lacustrine deposits remain fairly intact, though 

possibly concealed by volcanic and sedimentary units. In 

marked contrast, significant Late Cenozoic basin-and-range 

uplift and erosion in northwestern Nevada has disrupted the 

original basins considerably, and synchronous sedimentation 

into intermontane basins has concealed many earlier lacustrine sedimentary deposits that lie beneath the basins. Basin 

and lake reconstruction in this subregion likely would be 

extremely difficult. Indeed, Bradbury (1999) pointed out that 

the scattered remnants of the original lakebeds limit studies of 

basin morphologies and, thus, of the context of the diatomite 

deposits in a basinwide scenario. Tephrochronologic studies, 

such as those by Perkins and others (1998) and Perkins and 

Nash (2002), suggest a temporal continuity between some of 

these isolated basin remnants.

The amount, style, and timing of uplift and erosion 

profoundly influenced diatomite deposits. As shown in the 

Brady area (Stewart and Perkins, 1999), uplift began during 

lacustrine sedimentation, sedimentary materials stripped from 

the highlands inundated the lake, and normal faulting began 

to modify and segment the basin. Uplift that immediately 

follows sedimentation also influences diagenetic and postdiagenetic alteration by eliminating the lake and modifying the 

ground-water table. In most places, this influence is beneficial economically because it limits alteration of the diatoms, 

retains the original diatom textures and purity, and permits 

oxidation and removal of organic materials. However, poorly 

cemented lacustrine sedimentary materials are more susceptible to erosion than more resistant rocks, such as volcanic 

Postdepositional Events
10 Contributions to Industrial-Minerals Research 

flows. These materials generally accumulate in topographic 

lows that are closer to the water table or more likely to be 

covered by later volcanic units or alluvium. Again, this influence may not be entirely detrimental; Nash (1995) noted that 

modest fault-related downdropping of diatomite and concealment by relatively thin alluvial deposits combined to preserve 

the diatomite deposits in the Trinity Pass area west of Lovelock, Nev. At the full extreme, extensive high-angle faulting 

and related erosion can completely eliminate a diatomite 

deposit or conceal it beneath extensive, thick alluvium. Given 

the high heat flow in the Great Basin region over the past 

6 m.y. (Lachenbruch and Sass, 1978), burial to 100–500 m 

could cause thermal recrystallization of the diatom frustules 

(Knauth, 1994; Nash, 1995).

Possible topics of study:

• In what tectonic environments and local structural 

 settings were diatomite deposits most likely to be either 

 preserved or destroyed by uplift and erosion?

• What effect does burial have on diatomite, includ-

 ing destruction of frustules and increased availability 

 to ground water?

Conclusions and Future Plans

This chapter introduces many, but not all, of the factors that led to the formation and subsequent preservation 

or destruction of diatomite deposits in Late Cenozoic lakes 

of the Great Basin region. Some of these factors are global 

to regional, such as climate and tectonics, whereas others 

are more local, such as volcanic flows and controlling structures. A favorable combination of these processes led to the 

presently exposed diatomite deposits in various parts of the 

region, and less than optimum combinations elsewhere prevented the formation of diatomite deposits or destroyed or 

buried them after formation.

The goal of the Industrial Minerals in Lacustrine Environments Task of WIMP is to study these processes in more 

detail. Several critical regional and local factors have received 

little or no research attention, and more information on some 

or all of these factors would provide a better understanding 

of diatomite deposits in the region. Regional factors include 

original distribution and extents of lakes, elevation variations, 

subregional climate variations, volcanic activity, and tectonic 

environment. Basin- and deposit-specific factors include 

duration and age of diatomite formation; paleoecology, paleogeography, and chemistry of the lakes; sources and amounts 

of silica and nutrients; diatom speciation and related flora and 

fauna; structural environment during and after diatomite formation; sedimentologic controls, including types and sources 

of clastic materials; causes of lake formation and termination; 

and postdepositional alteration. Using a combination of past 

and ongoing research in related fields and new work for this 

project, we expect to gain a better understanding of the processes that formed and preserved diatomite deposits. From this 

understanding, development of better regional exploration 

and assessment models for undiscovered economic diatomite 

deposits will be feasible, especially for those deposits that 

may be only slightly concealed.

Acknowledgments

Karen Bolm provided the locations of MRDS and MAS–

MILS diatomite deposits shown in figures 1 and 3. Unpublished data from geologists in the diatomite industry was used 

with their permission but was not referenced specifically. 

The manuscript benefited from reviews by Phil Moyle, Keith 

Long, and George Havach.

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